1. Field of the Invention
The present invention relates generally to the field of microphones and more particularly to a tooth bone conduction microphone method and apparatus.
2. Description of the Prior Art
Conventional (air-conduction type) microphones are routinely used for converting sound into electrical signals. One such application is the Phraselator that is currently used by Department of Defense. The Phraselator primarily consists of a microphone, an automatic speech recognition module, a language translator, and a voice synthesizer with a speaker. The English phrases spoken by the user is captured by the microphone and translated to other languages such as Dari (used in Afghanistan), and sent to a speaker, which announces the equivalent Dari phrase.
Although usable, the Phraselator is highly vulnerable to typical military noise environment resulting in degradation of its performance. The performance improves when the user holds the microphone very close to his mouth, however it still does not work all the time. The microphone, due to the presence of typical military environment noise, does not accurately capture the spoken words. Microphones pick up the acoustic signals coming from any direction from any source and cannot distinguish. Directional microphones are superior in applications if the source of the sound is always from the same direction. However, even the best directional microphones have limitations when used in military noise environment. Conventional microphones cannot differentiate between the human voice and any other environmental sound. They are unable to reproduce the spoken sounds faithfully. In addition, the reverberation of the spoken sound introduces additional complexity in conventional microphones by way of repeated sound waves. Therefore, there is an immediate need to develop a microphone or an equivalent module that is immune to the surrounding noise (military or otherwise) and has improved signal to noise ratio.
The action of speaking uses lungs, vocal chords, reverberation in the bones of the skull, and facial muscle to generate the acoustic signal that is released out of mouth and nose. The speaker hears this sound in two ways. The first one called “air conduction hearing” is initiated by the vibration of the outer ear (eardrum) that in turn transmits the signal to the middle ear (ossicles) followed by inner ear (cochlea) generating signals in the auditory nerve which is finally decoded by the brain to interpret as sound. The second way of hearing, “bone conduction hearing,” occurs when the sound vibrations are transmitted directly from the jaw/skull to the inner ear thus by-passing the outer and middle ears. As a consequence of this bone conduction hearing effect, we are able to hear our own voice even when we plug our ear canals completely. That is because the action of speaking sets up vibration in the bones of the body, especially the skull. Although the perceived quality of sound generated by the bone conduction is not on par with the sounds from air conduction, the bone conducted signals carry information that is more than adequate to reproduce spoken information.
There are several microphones available in the market that use bone conduction and are worn externally making indirect contact with bone at places like the scalp, ear canal, mastoid bone (behind ear), throat, cheek bone, and temples. They all have to account for the loss of information due to the presence of skin between the bone and the sensor. For example, Temco voiceducer mounts in ear and on scalp, where as Radioear Bone Conduction Headset mounts on the cheek and jaw bone. Similarly, throat mounted bone conduction microphones have been developed. A microphone mounting for a person's throat includes a plate with an opening that is shaped and arranged so that it holds a microphone secured in said opening with the microphone contacting a person's throat using bone conduction. Bone conduction microphones worn in ear canal pick up the vibration signals from the external ear canal. The microphones mounted on the scalp, jaw and cheek bones pick the vibration of the skull at respective places. Although the above-referred devices have been successfully marketed, there are many drawbacks. First, since the skin is present between the sensor and the bones the signal is attenuated and may be contaminated by noise signals. To overcome this limitation, many such devices require some form of pressure to be applied on the sensor to create a good contact between the bone and the sensor. This pressure results in discomfort for the wearer of the microphone. Furthermore, they can lead to ear infection (in case of ear microphone) and headache (in case of scalp and jaw bone microphones) for some users.
There are several intra-oral bone conduction microphones that have been reported. In one known case, the microphone is made of a magnetostrictive material that is held between the upper and lower jaw with the user applying a compressive force on the sensor. The teeth vibration is picked up by the sensor and converted to electrical signal. The whole sensor is part of a mouthpiece of a scuba diver.
Also, some experimental work has been done in using a tethered piezoelectric-based accelerometer mounted on teeth to measure bone conduction induced vibration and compared to standard signals. The accelerometer protruded through the lips making the approach difficult to implement in practice. The sensor is bulky and puts unbalanced load on the teeth making them useful only for experimental purposes, at the best. Therefore there still exists a need for a compact, comfortable, economical, and practical way of exploiting the tooth bone vibration to configure an intra-oral microphone and preferably wireless.